I live in the Great Sovereign State of Texas here in the US of A! As long as they have a helmet if under the age of 18, good to go!
I started riding my little girl (now 13 going on 25) on the back of the bike when she was about 3. Had to rig up a seat belt (old belt) and shoulder harness system to keep her on the bike. As soon as we started rolling, she would be fast asleep. With that big old helmet on, she would flop over left or right. With the rig I put on the bike, there was no worry about her actually falling off, but all the Soccer Moms in the SUVs would freak out at STOP lights or STOP signs frantically pointing at Asia and mouthing, "YOUR BABY IS FALLING OFF". I would wave back and wander off down the lane to the next panic point.
She is now way too blasé about riding on the bike. Old hat!! But I have a little guy that turns 3 in June!!!
TQ